In the usual lens system, there always arises color aberration caused by the refractive index. This will be explained with reference to FIG. 1. When a sensing device is disposed behind a lens L as shown, it is observed that the intensity of the blue and green components of light is higher on the lens side of the focal point f while the intensity of the red component of light is higher behind the focal point. To cope with this problem, it has been the practice to use an achromatic lens, use a monochromatic light source or perform compensation of the color aberration in the visible wavelength range.
In a sensing device for color distinction with such aberration compensation means, however, color errors are apt to occur depending on the location of the device. Further, with commercially available sensing devices for color distinction based on tristimulus values it is impossible to realize accurate color distinction of an object at a given measuring point because of the fact the individual sensing devices have different optical axes. Accordingly heretofore, the color distinction has been done by approximation.